We are using pmr resonances of modified bases in several tRNA species to follow changes in tertiary structure. Of particular interest are the effects of temperature, metal ion concentration, and oligonucleotide binding to particular regions of the tRNA's. These studies are supported by work with synthetic oligonucleotides containing modified bases. The other major effort in this project involves pmr relaxation studies of substrates, inhibitors, nucleosides and nucleotides interacting with aspartate transcarbamylase from E. Coli. These data provide the means of extracting rate constants, dissociation constants, pKa's and rotational correlation times of bound species in this allosteric system.